Remote keyless entry (RKE) circuits are extensively used in vehicles to provide a convenient way to lock and unlock vehicle doors. Current RKE circuits usually include a super-regenerative receiver or a super-heterodyne receiver to receive signals from a key fob. Super-regenerative receivers tend to have a wider bandwidth than super-heterodyne receivers and therefore admit more noise. However, super-regenerative receives are also less expensive, making them more desirable for meeting vehicle manufacturing cost requirements.
RKE circuits for receiving key fob signals draw current even when the vehicle is turned off because they need to be able to receive a key fob signal at any time and assess whether to unlock the vehicle door. The RKE circuit draws current from a main vehicle battery, which ideally has a minimized size to promote low cost and fuel economy. Thus, it is desirable for the RKE circuit to have a low biasing current draw. This may be difficult, however, because the RKE circuit has both a microcontroller and receiver that must be operating nearly continuously to detect the key fob signal, thereby constantly drawing current even when the vehicle is turned off.
The difference between the current draw of currently known RKE circuits and the desired current draw tends to be significant. For example, the microcontroller may draw a minimum of 10 mA and the receiver may draw 5 mA for a total current draw of 15 mA, which is significantly higher than the 2–3 mA current draw requirement for the entire electronic control unit (which includes the RKE circuit) in many design requests.
It is possible to reduce the overall current draw by switching the microcontroller and the receiver in the RKE circuit into a low-current sleep mode and periodically waking the microcontroller and the receiver to check for a key fob signal. The frequency and duty cycle of this wake function is dictated by a continuously running clock that draws very little current. In other words, the clock acts as a real-time interrupt counter for the microcontroller. Each time the counter overflows, the microcontroller wakes up, turns on the receiver, and then put itself back to sleep as the receiver operates for a selected finite timeframe. During the finite timeframe, the receiver will send any detected digital data bits to the microcontroller for evaluation.
Other methods include using a resistor-capacitor (RC) timing circuit connected to a transistor whose gate is connected to the capacitor. In this configuration, if a data bit of sufficient duration travels through the RC circuit, it will charge the capacitor, thereby biasing the gate of the transistor to turn it on and allowing current to reach the microcontroller to wake up the microcontroller. This method has the advantage of waking up the microcontroller on an as-needed basis to minimize current draw. However, large amounts of noise may also reach the microcontroller in a form that looks like data.
This noise is particularly pronounced when the microcontroller initially turns on the super-regenerative receiver, which creates a large stabilization pulse on the order of 3–4 ms long before any data is transmitted through the RC circuit. This stabilization pulse is large enough to look like data to the microcontroller and is substantially larger than a typical 500 microsecond data bit. Thus, if the microcontroller requires two data bits before responding (e.g., 1 ms worth of data) and the stabilization pulse is 3–4 ms, the time constant of the RC circuit must be adjusted to accommodate the stabilization pulse first and then any subsequent data.
During this time, the microcontroller remains turned on to wait for the stabilization pulse to pass before obtaining the data. Also, the RC circuit must have a time constant that is long enough to accommodate the stabilization pulse and to allow the microcontroller to obtain the data after the stabilization pulse. Keeping the microcontroller on for such a long period (i.e., the duration of the stabilization pulse and any subsequent data), however, undesirably increases current consumption of the RKE circuit.
There is a desire for an RKE circuit that is able to handle stabilization pulses that occur when the RKE circuit is cycled and retains the capability to resolve RF data transmitted while still keeping current consumption to a minimum.